Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/workbox-build/build/lib/additional-manifest-entries-transform.js

@@ -0,0 +1,47 @@
+"use strict";
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.additionalManifestEntriesTransform = void 0;
+const errors_1 = require("./errors");
+function additionalManifestEntriesTransform(additionalManifestEntries) {
+    return (manifest) => {
+        const warnings = [];
+        const stringEntries = new Set();
+        for (const additionalEntry of additionalManifestEntries) {
+            // Warn about either a string or an object that lacks a revision property.
+            // (An object with a revision property set to null is okay.)
+            if (typeof additionalEntry === 'string') {
+                stringEntries.add(additionalEntry);
+                manifest.push({
+                    revision: null,
+                    size: 0,
+                    url: additionalEntry,
+                });
+            }
+            else {
+                if (additionalEntry && additionalEntry.revision === undefined) {
+                    stringEntries.add(additionalEntry.url);
+                }
+                manifest.push(Object.assign({ size: 0 }, additionalEntry));
+            }
+        }
+        if (stringEntries.size > 0) {
+            let urls = '\n';
+            for (const stringEntry of stringEntries) {
+                urls += `  - ${stringEntry}\n`;
+            }
+            warnings.push(errors_1.errors['string-entry-warning'] + urls);
+        }
+        return {
+            manifest,
+            warnings,
+        };
+    };
+}
+exports.additionalManifestEntriesTransform = additionalManifestEntriesTransform;